1. Field of the Invention
This invention relates to motion adaptive image processing.
2. Description of the Prior Art
Video image capture represents a spatial and temporal sampling process. An image is captured as a set of pixels arranged in rows or lines. Successive images are captured at spaced instants in time.
A complication is the common use of interlaced video capture and processing. In an interlaced video signal, each image is handled as two sets or fields of alternate lines of pixels. For example, odd numbered lines might be included in one field, whereas even numbered lines could be included in the next field. An advantage of interlaced techniques is that they give an apparent doubling of the image rate, so reducing flicker effects, for no substantial increase in video signal bandwidth.
All of these aspects of sampling can give rise to alias effects if an attempt is made to capture or process video material having spatial or temporal frequencies which are too high for the respective sampling rate. But a particular alias problem will be described here in the area of interlace to progressive scan video conversion.
If it is desired to convert between interlaced video and progressive scan (non-interlaced) video, then for non-moving images it is merely necessary to interleave two successive fields to recreate a non-interlaced frame having all lines of pixels present. However, if there is any significant inter-field motion, this approach may not work. In such circumstances it can be more appropriate to derive the lines of pixels which are missing in one field from other pixels in that same field. In other words an intra-field interpolation process is used.
In practice, a video source may comprise image sequences in which some regions represent moving images whilst some regions do not. For example, when a newscaster speaks to a fixed camera, the newscaster's mouth, face, and head may move considerably, whilst their torso, the desk and the wall behind them do not.
Therefore the different conversion strategies noted above may be appropriate within different regions of the same image. It is therefore important to determine which strategy to use for a given pixel.
Interpolation will generally give a worse result than interleaving for non-moving portions, whereas interleaving and will generally give a worse result than interpolation for moving portions. So, the choice of the more appropriate technique is very important.
In particular, interleaving between image fields generally gives worse results than interpolation of a single image field when there are moving portions of the image because it results in a striped effect in the resultant image as alternate lines taken from different image fields will have visibly different content.
Moreover, under some circumstances it is also possible for successive fields to be visibly different despite a lack of substantial motion. In particular, when there is a sudden change in lighting (for instance, when a light is switched on in a room, or there is lightning or a strobe effect). In such circumstances it is also desirable to avoid interleaving between image fields within affected portions of an image.
It is an object of the present invention to seek to alleviate or mitigate this problem.